A random access procedure used in a wireless communication network allows wireless devices to obtain new or renewed access to the network and to obtain uplink time synchronization. There are different reasons for the wireless device to transmit a random access request, for example when desiring initial access to the network, or when it has been set out-of-synchronization in a downlink/uplink and need to perform a random access procedure to receive a downlink data transmission/send an uplink transmission or when there is an imminent handover.
The random access procedure may be either contention based or contention free. The former is applicable for all the mentioned cases, while the latter is applicable only to downlink data transmission and incoming handover.
In the contention based random access (CBRA) procedure, also denoted the general random access procedure, a number of preamble sequences are used in common by all wireless devices. The wireless devices can use any one of the preamble sequences and CBRA thus requires signaling to resolve any contention that may have occurred. The contention free random access (CFRA) procedure, also denoted the dedicated, the non-contention based or the contention less random access procedure, uses a preamble sequence that is dedicated for one wireless device to use.
The signaling between the wireless device and a radio access network (RAN) for the CBRA procedure is illustrated in FIG. 1 and described in the following for a Long Term Evolution (LTE) scenario. The wireless device (denoted UE in the figure) initiates the CBRA procedure by transmitting (arrow 1 in FIG. 1) a randomly selected CBRA preamble from within the CBRA preamble range which is received via System information block 2 (SIB2). SIB2 carries radio resource configuration information which is common for all wireless devices. The RAN, and in particular a eNodeB (evolved Node B, also denoted eNB) thereof, acknowledges the detected CBRA preamble by transmitting (arrow 2 in FIG. 1) a random access response message including among other things an initial grant to be used on an uplink shared channel, a timing advance (TA) update and a temporary cell-Radio Network Temporary Identity (C-RNTI). When receiving the response the wireless device uses the grant to transmit (arrow 3 in FIG. 1) a message that serves the purpose of verifying that the wireless device received the random access response and includes necessary information for the purpose of contention resolution. The RAN receives this message and performs contention resolution (arrow 4 in FIG. 1) based on information received in the message and eventually one wireless device will detect random access (RA) success while other wireless devices will roll back to do CBRA over again.
The wireless device can perform CBRA in all the four cases mentioned earlier. On the eNB side, when multiple preambles are received within the same Physical Random Access Channel (PRACH) occasion, a known method is to sort the preambles based on the received signal power and schedule the random access response transmissions (arrow 2 of FIG. 1) in accordance with this sorting.
Since the wireless devices are scheduled based on the received preamble power, a wireless device having less favorable radio conditions has to wait the longest for a random access response transmission. A wireless device in a handover situation often has such less favorable radio conditions, and may loose its connection to the wireless communication network. Operators of the wireless communication networks are well aware of the fact that users are typically more annoyed and dissatisfied when being disconnected during an ongoing call than when having to wait to obtain an initial access to the wireless communication network. There is thus a need for improvement in this regards.